The present invention relates mainly to the fields of optical communication, particularly to multiple wavelength sensitive elements known as wavelength demultiplexers or photoreceivers used for discriminating incoming light signals (optical channels) of different wavelengths and for subsequent forwarding individual optical channels to separate electrical outputs. It can also be used in some other areas such as optical imaging, spectrometry, photoelectric energy conversion, etc.
With the explosive growth in demand for tele-communication and data-communication bandwidth, network operators have turned to wavelength division multiplexing (WDM) as a means for increasing the capacity of fiber networks.
WDM is a process in which numerous signals are combined for transmission through a single communications line. Within a data batch each signal has a specific wavelength.
At the output of transmission lines the multiplexed optical channels need to be separated according to their specific wavelengths and forwarded to individual electrical outputs. Separation of optical channels by their wavelengths is accomplished by a device known as a demultiplexer, whereas conversion of optical signals into electrical signals is accomplished by a device known as a photodetector. In other words, a receiving part of each WDM system normally contains an optical demultiplixer and a photodetector line or array with individual photodetectors for individual wavelengths.
In existing WDM systems, a channel wavelength demultiplexing function is typically performed by specific optical devices known as Fiber Bragg Gratings (FBG) or Arrayed Waveguide Gratings (AWG), while individual channel detection and photoelectric conversion are done by means of PINxe2x80x94photodiodes or avalanche semiconductor photodetectors (APD) [see, e.g., xe2x80x9cUnderstanding Optical Communicationsxe2x80x9d by H. J. R. Dutton, 1998]. In other words, any optical communication system normally has at least two separate groups of devices to carry out the end-point channel analysis (i.e., channel demultiplexing and photoelectric detection).
It is also important to note that existing WDM systems typically use certain spectral ranges to transmit multi-channel optical data. Most popular spectral ranges have a spectral width of 50-100 nm and are located in the vicinity of 1310 nm and 1550 nm wavelength, respectively. It is expected, however, that significant expansion of the useful spectral range will become available in coming years so that future fiber optical communication (FOC) systems will be able to transmit data in wider spectral ranges, e.g., 1200-1700 nm, with a corresponding increase in the number of optical channels. To meet such demultiplexing and photoelectric conversion requirements for normal operation conditions is a challenging task. Therefore a demand may occur for new demultiplexing and photoelectric devices as well as for various combinations of xe2x80x9cwide-spectrumxe2x80x9d demultiplexers and photoreceivers with known optical devices capable of working in the aforementioned spectral ranges.
Attempts have been made heretofore to combine wavelength demultiplexing and photodetecting functions. Short review of some of such devices is given below.
U.S. Pat. No. 4,514,755 to Tabei discloses a visible color imager wherein partially absorptive layers for blue, green, and red portions of incoming light are incorporated into a MOS/MNOS switching array. However, this device is not applicable to FOC systems because it utilizes spectral ranges different from those in the existing FOC systems and does not possess wavelength discrimination resolution required for such systems.
U.S. Pat. No. 4,613,895 to Burkey B. C. et al. discloses a color responsive image device employing wavelength-dependent light absorption to separate colors in a semiconductor device consisting of a plurality of alternately doped layers. This device entails the same disadvantages as the previous one. Furthermore, if such a device is used in a FOC system, it will have a noticeable optical channel interference (channel cross-talk) because the doped layers of this device will absorb a significant part of light energy from neighboring optical channels.
U.S. Pat. No. 4,975,567 to Bishop S. G. et al. discloses a multi-band quantum well-type photodetector comprising a plurality of GaAs/AlGaAs layers of alternate thickness. Although this device is applicable to FOC systems, it is characterized by very strict requirements to the thickness of individual quantum layers. Furthermore, for better performance this device needs a strictly controlled cooling system.
U.S. Pat. No. 5,138,416 to Brillson L. J. and U.S. Pat. No. 5,298,771 to Mantel D. disclose multilayer imageries comprising pluralities of III-V semiconductor layers made of selected alloy compositions in order to separate and consequently detect the blue, green and red colors in series in optical imaging devices. Along with drawbacks inherent in some of previously described multiplexing devices, the devices of U.S. Pat. No. 5,138,416 and U.S. Pat. No. 5,298,771 utilize a sequential procedure for color channel read-out. This principle is inapplicable to FOC systems, as it will slow down a bit-rate thus limiting an operating speed as well as optical network bandwidth.
U.S. Pat. No. 4,513,305 to W. L. Bloss et al. discloses a multi-wavelength demultiplexer comprising a plurality of III-V semiconductor layers made of selected alloy compositions and energy gaps in order to separate and consequently detect different spectral portions of incoming radiation thus performing combined demultiplexing and photoelectric conversion functions in the same monolithic device. However, the device of U.S. Pat. No. 4,513,305 has the a number of disadvantages which are described below. In particular, it requires the use of relatively thick photosensitive layers combined with the wide-gap buffer layers which increases the total number of layers as well as the thickness of the device as a whole. This contributes to complexity of the device structure and makes it expensive to manufacture. Optical channel photodetection from a single photosensitive layer of large length by means of two side contacts can not be made fast enough for a modern FOC system due to the finite electron-and-hole drift time through the layer. Two-sided connection to each photosensitive layer makes the design more complicated and the operation less reliable.
It is an object of the present invention to provide a semiconductor wavelength demultiplexer which is simple in construction, reliable in operation, is capable of discriminating a number of wavelengths, may efficiently utilize layers of different energy gaps with reduced channel xe2x80x9ccross-talkxe2x80x9d, does not slow down a bit-rate and hence does not limit an operating speed as well as optical network bandwidth, can perform well without strict thickness control, and does not require cooling, makes it possible to reduce number of optical components necessary to demultiplex and detect an optical signal, allows for an usage of wider transmission spectral range, and allows for a decrease in the thickness of the layers.
In accordance with the present invention, there is provided a solid-state wavelength demultiplexer comprising a plurality of photosensitive semiconductor compound layers wherein each layer has a certain energy gap defined by the composition of its material. All layers are grown on a common substrate where the first grown buffer layer, adjacent and near the lattice matched to the first bottom photosensitive layer, is heavily doped. Compositions of photosensitive layers vary from the lowermost photosensitive layer to the uppermost photosensitive layer in such a way that a corresponding energy gap has the minimum value in the lowermost layer and the maximum value in the uppermost layer. A wide-gap xe2x80x9cwindowxe2x80x9d layer is grown on the top of the uppermost photosensitive layer. An anti-reflection coating may be deposited on top of the xe2x80x9cwindowxe2x80x9d layer.
Each photosensitive layer of a given energy gap, in turn, comprises a heavily doped sublayer of one type of conductivity, adjacent to the next upper photosensitive layer (the one with a wider energy gap), followed from the bottom by an undoped sublayer in which light induces a carrier generation process. Furthermore, an electron-hole separation process takes place mainly in this sublayer. The aforementioned undoped sublayer is followed by another doped sublayer which is adjacent to the next lower photosensitive layer and which has conductivity of a type opposite to that of the aforementioned heavily doped sublayer. The demultiplexer is further provided with a common heavily doped layer of the same type of conductivity as the aforementioned lower doped sublayer in each of the photosensitive layers. This common layer is electrical contact to all aforementioned lower doped sublayers. Furthermore, a plurality of etched-off steps and contact regions are formed on one side of the device to provide electrical outputs to all photosensitive layers through the upper doped sublayers. The demultiplexer of the present invention makes it possible to reduce channel power loss and eliminate inter-channel xe2x80x9ccross-talkxe2x80x9d while maintaining high quantum efficiency for optical channel detection.